Analog Avenue

Product of the Week

Archive | Feedback

Analog Devices Introduces First Integrated IF-to-Baseband Diversity Receiver for GSM/EDGE Wireless Base Stations

New MxFE device integrates the functionality of seven components on a single chip.

The manufacturer says . . .

ChipCenter's Paul O'Shea says . . .

Analog Devices Inc., a global leader in high-performance semiconductors for signal-processing applications, introduced the industry's first integrated IF-to-baseband diversity receiver for GSM/EDGE wireless base stations. Combining the functionality of seven components on a single chip, this new receiver enables base-station designers to meet the strict EDGE (enhanced data rates for global evolution) performance requirements at 50 percent of the cost of existing solutions. The new device joins the growing family of MxFE (mixed-signal front-end) ICs, which leverage ADI's high-performance data converter and radio capabilities. "Our expertise in integrating multiple functions onto single chips allows us to answer the market need for an advanced-functionality, low-cost, single-chip receiver," said David Robertson, product line director for high-speed converters in communications, Analog Devices. "With our broad signal-processing technology portfolio, Analog Devices is in the unique position to innovatively partition the receiver signal chain, resulting in a solution that improves the functionality while lowering the cost for GSM/EDGE equipment makers." ADI also recently announced a chip-set solution for designing subscriber terminals that are compatible with the EDGE standard. The new diversity receiver, the AD6650, is a member of ADI's MxFE family, and is based on the company's "smart partitioning" methodology—a mixed-signal design technique that partitions the signal path according to performance-enhancing, rather than analog/digital, boundaries. As such, this device replaces five active components and two SAW filters in existing solutions, reducing both the board size and test costs. In addition, the new chip integrates a digitally controlled VGA (variable-gain amplifier) that acts as part of an on-board automatic gain control loop, an IF-to-baseband I&Q demodulator, analog low-pass filtering, and dual A/D converters. An on-chip synthesizer with an integrated VCO (voltage-controlled oscillator) drives the I&Q demodulator. The part also integrates digital decimation filters, which generate serial-output data streams and remove unwanted signals and noise outside the channel of interest. The AD6650 can accommodate IF input frequencies from 70 MHz to 300 MHz, and only requires one external SAW filter for the entire Rx signal path to meet GSM/EDGE blocking requirements. Analog Devices is a leading manufacturer of precision high-performance integrated circuits used in analog and digital signal-processing applications. The company is headquartered in Norwood, Massachusetts, and employs approximately 8,600 people worldwide. It has manufacturing facilities in Massachusetts, California, North Carolina, Ireland, the Philippines, Taiwan, and the United Kingdom. Analog Devices, Inc. Ray Stata Technology Center 804 Woburn Street Wilmington, MA 01887 Tel: 1-800-ANALOGD      (1-800-262-5643) Web site: www.analog.com

According to Analog Devices, the AD6650 is an industry first, integrating the IF-to-baseband receiver for GSM/EDGE markets. The company uses their smart-partitioning technology to help make this integration possible. Essentially, the technology allows digital and analog functions to coexist on the same chip. For example, if you have an analog-to-digital core, many of those cores would be divided, especially if they were developed on a 0.25 µm process technology. Typically, it doesn't make much sense to incorporate digital logic on 0.25 µm technology because it's not efficient for digital. That's where the company's idea for smart partitioning came into play because manufacturers wanted some digital capability onboard even if it didn't make sense to integrate the analog with all the digital functions. Total integration of analog and digital becomes less viable especially as the designs move down the scale from 0.18 µm to 0.13 µm. There are technical challenges that arise with a smaller process that make it more convincing to have a separate digital block—and the smart partitioning can help. The AD6650 is a significant product for several reasons, and the integration capability is certainly one of the important reasons. Simply put, systems manufacturers need this type of product to meet the upcoming demand for GSM/EDGE and to control their costs. The GSM/EDGE market is expected to increase in popularity and peak about 2004, according to some analysts, before other technologies start replacing it. The systems companies are focused on cost-cutting measures, and ADI offers them a good solution for the GSM/EDGE market. Many of the products being deployed now by companies like AT&T, Cingular, and VoiceStream are considering and probably implementing base stations that are EDGE-compliant. These operators want to take advantage of the data services provided by EDGE. ADI expects that the North American market will probably see the highest rate of deployment for EDGE in the next few years, then peak around 2004, and be around for another 8 to 10 years. So this chip makes good sense for the burgeoning market. There have been other attempts by several companies, including Analog Devices, to design a better mousetrap for the GSM/EDGE market. For example, the first attempt by ADI used an off-the-shelf solution that got the job done, but maybe not as efficiently as some would like to have seen. The second generation saw the development of IF sampling, thus reducing parts counts, and the use of Application Specific Standard Products or what some call ASICs made for OEMs. The newest or third-generation product integrates IF-to-baseband, which includes the variable-gain amplifier (VGA) that is part of the digitally controlled automatic gain-control loop. The design also has active mixers for demodulation—to mix down to the baseband frequency. Next, there are low-pass filters and two 12-bit A/D converters that use two receive paths and process the information on-chip. A multiplexer is used to simultaneously sample the IQ data stream and interleave that data through the A/D converter so the design can use two A/D converters for the main and diversity receive signal paths, compared to the previous baseband designs that used four A/D converters. click for full-size image ADI says they are not aware of anyone who has integrated this much, analog or digital, onto a single chip for this function, and that appears to be correct. In addition to the integration, some innovations are on the digital back-end. The first is the AGC loop. Typically, you would use two SAW filters in a receive path, but this chip only requires one SAW filter per path. So instead of having four SAW filters in the main and diversity receive signal paths, you only need two filters—one for the main and one for the diversity path. The reason ADI can do that is because of the AGC loop and because the chip has enough dynamic range to get the performance without a second SAW filter. The AGC loop is very flexible, offering three programmable modes. One of the modes the company calls the Slow Attack. This is important for signals that experience a slow fade that occurs when, for example, the cell-phone user drives behind a building and the signal to the tower gets weaker. As the signal slowly weakens, the VGA increases the gain to the base station so it looks like there is still a good signal. That means that the AGC is continuously monitoring the output to the A/D converter, and if the desired signal starts getting small, a decision block is used to increase the gain. The opposite is true when you need to reduce the gain. Another mode is the Fast Attack mode. For example, within one 200 kHz carrier you have eight time slots, and each represents a user. One user may be far away from the tower so that signal will be small. Another may be close and have a large signal. The base station sees that first person's signal as small, so the gain on the VGA would be very large because it needs to amplify the signal to receive it. When the system switches time slots for the second user who is close, the gain is now too large. If the system drives a large signal, it will overdrive the part and ruin the signal. ADI provides a method to reduce the gain quickly, and they call it the fast attack method. It keeps the ADI chip with the optimum dynamic range even when moving to different time slots. This is important, and many base-station companies will be interested in it. The third attack mode is the Fast Decay mode, where the users are close in the first and far away in the second time slot, causing the system to respond quickly when moving from time slot to time slot. Something else that could be missed but is very important with this generation of chip is the capability for DC correction. In the previous-generation chip, DC correction was not an issue because the chip used IF sampling, and the DC got mixed away when it went through the mix-down stage. However, with a baseband architecture like the first-generation chip, everything beyond the mixers moves all the way through the path and can upset the signal you can receive. So ADI had to compensate digitally for the DC that came through from the converters and filters. The company designed a circuit that would null out the DC while keeping the desired signal. To conclude, this solution compares very favorably to most existing solutions. For example, there is a five-chip solution available that can be replaced by this single-chip solution. Additionally, the 6650 replaces the two SAW filters in the receive path. It also has a large IF range extending from 70 MHz to 300 MHz. The AD6650 is sampling now, and will be in full production in the fall of 2003. It is available in 121-pin plastic ball-grid array (BGA) packaging. The AD6650 is priced at $20.00 per unit in 1,000-piece quantities. Data Sheet

Analog Main | Product of the Week | Columns | Editorial | Tech Notes

 

		Click here to get your listing up.